STEP 1create the 3D model in max. It is best to import an item file first so you know what scale to work on. Also, when making new walls you have to import the original walls because you cannot change wall heights or dimensions (atleast not until the guys from Zhanghuo TW finally release their tool to the public...)

STEP 2export the model(s) in cas format (not item). Put them in the data/models_building folder

STEP 3run the db editor. I'm not going in detail on how to use this. What you have to do is add a new model entry (or overwrite an existing one), setting your new 3D models in the LOD slots. The values in the boxes next to the LOD slots have to be a sequence of the values "0.00 10.96". The last values in this sequence are the LOD distance values. For example of you have 3 LOD models it will be:0.00 10.96 0.0010.96 0.00 10.96 <= repeating sequence100.00 500.00 4000.00 <= last 3 values are LOD distances

STEP 4define your new item entry in one of the files in data\descr_building_battle (or create a new file, but then it must first be included in data\descr_building_battle.txt).

STEP 5use your new item entry in a settlement plan. As soon as you start a game using this plan the game will create an item file from the cas LOD levels (which then become obsolete).

NOTE: changing physical dimensions of battlemap models is not yet possible. A modeller from the Zhanghuo mod is working on a tool for this (and might already have completed it) but so far he refused to release this tool until they have completed their mod. Let's be happy people like Vercingetorix do not behave like this or the only RTW mods we'd have by now would be edited skins.

1. Extract the file you need (BI or 1.5) to your data directory2. Backup descr_items.txt3. Rename the file you extracted to descr_items.txt4. Add to the beginning any Includes you want made, I recommend using this to add new buildings (you can see a readme on that in the original descr_items.txt)5. Add your new building entries6. Delete descr_items.db7. Start the game8. You get tons of error message (normally if you use show_err), either click through them or just hit ctrl+alt+del and kill the game process, but descr_items.db is generated anyways9. Restart the game and enjoy

If you want to use that in a mod folder, you have to extract the RTW item files you want to use there in order for the game to find them.If you don't want to use a building that is in that file, just delete the entry.

Get the file here: http://www.twcenter.net/forums/attachment.php?attachmentid=695&stc=1